Most imaging interferometers (a) divide measuring beams into object beams that encounter test objects and reference beams, (b) recombine the object and reference beams, and (c) image the test objects with the recombined object and reference beams to form interference images of the test objects. Frequency-shifting interferometers exploit a recognition that interference phases of individual points within the interference images vary with the changes in measuring beam frequency at rates proportional to local optical path length differences between the object and reference beams. Intensity variations of corresponding image points within a plurality of interference images captured at different beam frequencies are evaluated to determine the rates (i.e., frequencies) at which the phases of the image points cycle through conditions of constructive and destructive with changes in beam frequency. The proportional optical path length differences associated with the determined frequencies of phase change for a plurality of image points can be assembled into optical profiles describing physical characteristics of individual test objects, such as surface topologies or optical thickness variations.
Unlike most other interferometers, which compare interference phases between different points within the same interference images for calculating relatively smooth optical profiles of the test objects, frequency-shifting interferometers compare the interference phases between the same points within different interference images formed at different measuring beam frequencies for calculating optical profiles over a greater range of greater point-to-point variation. To achieve desired accuracy over a significant range of point-to-point variation, however, frequency-shifting interferometers capture relatively large numbers of interference images at different beam frequencies. Examples are known in which 16 interference images have been captured for measuring smooth optical surfaces with continuous profiles while 128 interference images have been captured for measuring machined parts with more irregular profiles.
Algorithms for converting intensity data of individual points within a plurality of the interference images into rates of phase change can be simplified by evaluating interference patterns generated at equally spaced beam frequencies. Two different approaches have been used for producing successions of equally spaced beam frequencies. One approach linearly varies the spectral output of light sources over a continuum and captures interference images at equally spaced intervals of time. Another approach tunes the spectral output of light sources to discrete frequencies that are equally spaced. The former approach lacks accuracy because most light sources are not linearly variable over the required bandwidth because of various systematic, environmental, or other influences. Higher accuracy is achieved by monitoring the actual beam frequencies and employing a more complicated algorithm. The second approach is more time consuming and subject to noise from vibrations and temperature shifts. Extra time is required because the light sources must be adjusted to and stabilized at each discrete beam frequency. The tuning steps generate vibrations and measuring conditions, such as temperature, tend to drift over the extended period of measurement.